Process for producing porous ceramic filter for filtering of particulates from diesel exhaust gases

ABSTRACT

A process for producing a porous ceramic filter for use in the filtering of particulates from diesel exhaust gases, and the filter so produced, in which a foamable ceramic composition based upon an aluminosilicate hydrogel binder is expanded into a self-supporting, open-celled porous body of desired shape by virtue of in situ reaction between components of the composition, and thereafter treated to substantially reduce its alkali metal content and fired to produce ceramic bonds, the process further preferably providing on the intended outlet surface of the filter a thin porous ceramic membrane layer whose pores have an average diameter less than that of the pores within and at other surfaces of the ceramic filter.

BACKGROUND OF THE INVENTION

The present invention relates to porous ceramic articles, and moreparticularly to porous ceramic articles for use as filters for removingparticulates from diesel exhaust gases.

"Diesel exhaust traps" are filtering devices designed to removeparticulate material (e.g., soot) from the exhaust of automobile ortruck diesel engines, a need dictated to a large extent by increasinglystringent governmental regulations in the United States and Europeancountries regarding maximum allowable particulates in automotiveemission gases. Generally, soot trapped by the filtering devices is thenperiodically combusted in the filter so as to regenerate the filteringsurfaces, the combustion being initiated, for example, by electricalmeans or fuel burner devices associated with the overall trap design, orby variable operation of the engine itself or other means to provide tothe filter an exhaust stream sufficiently hot to initiate the combustionprocess.

As a consequence of the generalized designs for particulate traps ofthis type, the filtering element is required to have a number ofproperties. Obviously, it is essential that the filtering elementexhibit porosity which permits trapping of particulates, but at the sametime it is essential that the construction of the filter be such thatexhaust gases can travel relatively easily therethrough without creationof any significant degree of back pressure. Moreover, it is necessarythat the filtering element present a substantial filtering surface perunit length, area or volume so as to permit the element to be fabricatedin an overall size consistent with the constraints imposed by the sizeof the exhaust systems of diesel-powered automobiles and trucks. Becauseof its exposure to hot exhaust gases, and even higher temperaturesduring a combustion/regeneration cycle, the filtering element alsonecessarily must possess structural and dimensional stability under suchconditions.

The prior art has sought to provide filtering elements for dieselparticulate traps possessing these physical characteristics by resort toa variety of materials. Early efforts relied upon stainless steel meshor coils of fibrous metallic wire as filtering materials, as evidencedby U.S. Pat. Nos. 3,937,015 and 4,270,936, respectively. More recentefforts have concentrated upon ceramic materials since they generallypossess excellent structural and dimensional stability under stringent(i.e., high temperature) operating conditions, with the requirement thatthe trap exhibit porosity effective to filter soot from exhaust gasesbeing accomplished by various compositional and processing techniques.Most notable in these efforts has been the utilization of so-calledceramic monolithic honeycomb filtering elements as described, forexample, in U.S. Pat. Nos. 4,276,071 and 4,364,761 assigned to GeneralMotors Corp.; U.S. Pat. Nos. 4,329,162; 4,415,344; 4,416,675; 4,416,676;4,417,908; 4,419,108 and 4,420,316 assigned to Corning Glass; and U.S.Pat. Nos. 4,283,210; 4,293,357; 4,307,198; 4,340,403; and 4,364,760assigned to NGK Insulators. Essentially, these elements consist of amonolithic ceramic having a multitude of internal parallel chambersseparated by thin porous ceramic internal walls, with a number of thechambers being end-sealed so as to force particulate-containing exhaustgas to traverse across a porous wall before exiting the element.Generally, these elements are formed by an extrusion process, andmaterials are included in the compositions which are burned out duringthe firing process so as to provide the requisite porosity in theinternal filtering surfaces. In another process, as reflected in U.S.Pat. No. 4,363,644 assigned to Nippon Soken, foamed, structuralpolyurethane systems are utilized in admixture with inorganic materialsin processes wherein the polyurethane burns out during firing so as toleave behind a ceramic structure having a variety of interconnected opencells for trapping particulates.

While the structural and dimensional properties of ceramics generallylend themselves well to utilization as the material from which filterelements for diesel traps are constructed, it is not an easy orinexpensive matter to achieve from ceramic materials elements possessingthe porosity needed to effectively and efficiently filter soot as wellas permit exhaust gas flow without substantial build-up of backpressure. For example, in the highly permeable reticulated foam filtersin the art, a condition can occur ("blow-off") in which soot alreadycollected in the filter can be dislodged as a consequence either ofexcessive build-up or sudden increase in the velocity of the exhaust gasstream. Of additional importance, efforts toward optimizing the geometryof diesel filter trap designs (so as to facilitate inclusion of thefilter in the exhaust area of a vehicle, or to maximize filtration, orto facilitate regeneration or removal of the filter element) can beseverely limited by the inability to produce such shapes efficiently (orat all) utilizing ceramics.

In response to the foregoing limitations in current technology, thepresent invention provides compositions and methods for making porousrefractory ceramic filters for use in removing particulates from dieselexhaust gases. As will be seen, the invention employsmoldable/extrudable compositions which enable any variety of shapes tobe provided, methods for rendering those shapes more refractory andresistant to thermal shock upon firing, and methods for providing thinporous ceramic membrane layers on the fired article to enhance filteringability.

SUMMARY OF THE INVENTION

According to the present invention, there is provided a ceramiccomposition which is capable of being molded (e.g., by casting,injection-molding or by extrusion) into a desired configuration, andwhich develops predominantly open-celled porosity as a result ofinternal reactions between and among deliberately preset elements of thecomposition. Among the significant advantages of the composition is theability to manipulate its elements and/or the amounts thereof so as tocontrollably achieve a wide variety of characteristics in the finalceramic article. The moldable composition is self-setting, againcontrollably, and in the set state is then further processed to removetherefrom substantially all alkali metal in order to enhance and improvethe refractoriness and thermal shock properties of the fired porousceramic filter. During processing, steps also preferably are taken toprovide, on, e.g., the outlet surface of the fired porous ceramicfilter, a thin porous ceramic membrane.

The composition is comprised of an admixture of an aluminosilicatehydrogel and suitable refractory ceramic materials, e.g., refractoryoxides, carbides, nitrides, borides, silicides and the like, such asalumina, chromia, zirconia, magnesia, titania, silica and mixturesthereof (either as admixtures per se or admixed as part of a compounditself, e.g., mullite, cordierite, calcined kyanite and the like),silicon carbide, silicon nitride, boron carbide, boron nitride, and thelike. Also included as part of the composition are a particulate metal,a surfactant system and a gel strengthening agent, these latteringredients being present in essential yet relatively minor proportionsrelative to the hydrogel and ceramic components. In addition, refractoryfibers may be included in the composition to attain yet additionalstrength in the eventual molded and fired ceramic.

The foregoing composition is described with reference to generallyidentifiable constituents of the composition at the time of its moldingor extrusion and setting, but the general process of preparing thecomposition utilizes more fundamental components which, upon admixture,result in the formation of the described hydrogel (i.e., rather thanaddition of the hydrogel as an identifiable separate ingredient). Thehydrogel is formed from a water soluble source of silicate and a watersoluble source of aluminate, and the remaining components of thecomposition (e.g., refractory ceramic materials, surfactant, gelstrengthening agent, particulate metal, refractory fibers) can be addedto or distributed between one or both of the aluminate or silicatecompositions. Upon admixture of these two separately prepared andmaintained aqueous compositions or slurries, there is formed analuminosilicate hydrogel which serves to bind together all components ofthe composition. The hydrogel binder is self-setting at ambientconditions and is capable of setting and binding the composition to agenerally self-supporting structure within a brief but controllabletime.

According to the present invention, the component parts of the ceramiccomposition are admixed to form a moldable composition, generally addingthe silicate slurry to the aluminate slurry. Mixing is done in thisfashion because the aluminate slurry acts to retard the gelation timewhile the silicate slurry acts as an accelerator. Therefore, if thealuminate slurry is added to the silicate slurry, the possibility existsfor a rapid, but partial gelation to occur which would, in effect,result in an incompletely set mix. Before any substantial self-settingof the composition occurs, it is molded to the desired shape, takinginto account the fact that the composition will foam and expand toassume the final desired molded shape. During the self-setting reaction,additional reaction takes place within the composition in which theparticulate metal reacts with alkali (e.g. sodium) compounds in thecomposition to produce, inter alia, hydrogen gas. By arranging theself-setting hydrogel reaction to be of suitable duration, the moldedcomposition increases in volume as a consequence of the internal gasgeneration and takes on a porous nature as the gas evolves within andfrom the composition. Then, as close as possible to the cessation of gasevolution, the in-situ hydrogel formation causes the composition to setin the desired porous molded configuration.

The supportable porous ceramic shape prepared in this manner isthereafter treated to remove alkali metal therefrom in order to improverefractoriness and thermal shock stability in the fired porous ceramicfilter. This is effected through treatment of the porous shape withwater to remove leachable alkali metal compounds, followed by treatmentwith a dilute aqueous solution of an ammonium salt, preferably ammoniumchloride, to effect exchange of ammonium ion for remaining alkali metalions.

At particular points in the overall manufacturing process, stepspreferably are taken to provide on, e.g., at least the outlet side ofthe final fired porous ceramic filter, a thin porous ceramic membranelayer, i.e., a thin layer having open-celled porosity and whose poreshave an average size less than that of the pores at other portions ofthe filter. A number of means are provided to achieve this end, thepreferred being those wherein the membrane is formed in situ during theformation of the supportable porous shape.

DETAILED DESCRIPTION OF THE INVENTION

The aluminosilicate hydrogel portion of the composition used in thepresent invention is, in essential respects, as described incommonly-assigned U.S. Pat. Nos. 4,357,165 and 4,432,798, both of whichare expressly incorporated herein by reference. As described in thosepatents, the hydrogel results from the admixture of water solublesources of both silicate and aluminate (typically, sodium silicate andsodium aluminate), which admixture then self-sets at ambienttemperatures in times which can be exceedingly short (e.g., on the orderof as little as a few seconds but typically on the order of a fewminutes), but nevertheless can be controlled by predetermined choice ofmolar ratio between aluminate and silicate, concentration of water, andtemperature. The ability to exercise control over setting times for thehydrogel binder leads to important advantages with respect to attainmentin the present invention of molded ceramic filters of both desiredgeometry and desired porosity. Also described in the above-noted patentsis the utilization of the hydrogel components along with granularrefractory particles to produce, e.g., molds, by virtue of theself-setting hydrogel serving to bind the granular materials into aself-supporting structure.

According to the present invention, the separately prepared and admixedcomponents for forming the aluminosilicate hydrogel have added to themand/or distributed between them the remainder of the components whichwill make up the moldable ceramic composition and the eventual firedporous ceramic shaped filter article. As earlier noted, the essentialelements of this composition, besides the hydrogel-forming constituents,are refractory ceramic materials, particulate metal powder, a gelstrengthening agent such as silica fume and a surfactant component, withrefractory fibers or other conventional materials optional. Therefractory ceramic materials generally will be present in the overallcomposition in a weight percentage of from about 50% to about 90%,preferably from about 60% to about 70%. In a preferred embodiment of theinvention, the ceramic materials included in the composition will bechosen from cordierite, calcined kyanite and mixtures thereof, with mostpreferred compositions containing nearly equal weight proportions ofboth cordierite and calcined kyanite, e.g., from about 30 to 35% of eachceramic.

According to the invention, the requisite open-celled porosity in thefinal ceramic filter article is provided as a consequence of in situreaction between metal powder and alkali compounds (e.g., sodiumhydroxide) present in the moldable composition, resulting in developmentof hydrogen gas as a reaction by-product. As a consequence of thisinternal gas production and evolution, the composition will expand involume in the mold (or during extrusion as the case may be) and developporosity, the quantity of the composition obviously being regulated totake into account the expected (and predetermined) degree of expansionwithin the mold or during extrusion to arrive at the desired finaldensity and size of the article. At the same time, the surfactantpresent in the composition serves to break up the bubbles of evolvinggas in the aqueous composition to achieve, controllably, suitably smallbubbles and to assure that the porosity developed in the structure willbe predominantly of the open-celled type.

The preferred particulate metal is aluminum, although other metals ormetal alloys such as silicon or ferrosilicon which similarly will reactwith alkali compounds present in the composition to produce hydrogen gasalso can be employed.

For most generalized compositions, the amount of surfactant and metal(e.g. aluminum) powder will be relatively small compared to the othercomponents of the system, with the typical levels of addition of thesurfactant being in the range of from about 0.05 to 1.0 percent byweight of the total composition and the metal being in the range of fromabout 0.05 to 0.5 percent by weight of the total composition. Preferredranges of addition for these materials are 0.4 to 0.8 percent by weightfor the surfactant (most preferably about 0.6%) and 0.1 to 0.2 percentby weight for the metal powder (most preferably about 0.15%), and apreferred ratio between the surfactant and metal powder is generallyfrom about 2:1 to 8:1, most preferably about 4:1.

Among the preferred class of surfactants (which may be used alone or incombination) for use in the invention are the silicone glycols such asare available from the Dow Chemical Company for use, e.g., in producingpolyurethane forms. These surfactants have a stabilizing effect on thegaseous by-products produced and are available in a variety ofcustomized formulations (based upon the silicone glycol chemistry) thatare designed to control bubble (or cell) size as well as to dictate thatthe cells will be predominantly open. For example, the surfactants fromDow Chemical known by the tradenames DC 190, DC 198, Q2 5160 and Q25125, provide a mostly open cell structure in the present invention.Although the silicone glycol type surfactants are preferred, a varietyof other non-silicone surfactant types also may be employed, such asthose available from Air Products & Chemicals, Inc. under tradenameLK-221 and LK-443.

With respect to the aluminum or other metal powder, the average particlesize of the powder employed generally will be in the range of from about1 to 44 μm, and preferably about 6-9 μm, with the understanding that thelarger the surface area of the metal present in the composition, themore vigorous and extensive will be the foaming reaction.

Another essential ingredient of the composition of the invention is agel strengthening agent, preferably silica fume, although other suitableagents may be employed. Silica fume is a by-product collected in theairstream during the reduction of silica sand by coal or coke in an arcfurnace to make metallurgical-grade silicon metal. The particulates arehollow spheres, roughly 0.25 micron in diameter, composed of about 96%silica and having a light carbonaceous layer on their surface. Althoughthe mechanism by which silica fume operates in the compositions of theinvention is not entirely understood, its addition brings about a numberof advantages, such as lowering the viscosity of the composition for agiven solids content and reinforcing the gel network (without increasingviscosity) to give greater green strength. Without the presence of thesilica fume, the hydrogel bonded aggregate structure appears more proneto cracking during drying operations. By reinforcing the gel structure,the silica fume reduces shrinkage as the molded article is dried.Generally, it has been found that the silica fume is effective at levelsof from about 0.25 to about 10 percent by weight of the totalcomposition, preferably from about 1 to 4 percent by weight, and mostpreferably from about 1 to 2% by weight.

As noted, gel strengthening agents other than silica fume can beemployed, such as fly ash, manganese oxide fume, ferrosilicon fume andthe like. Based upon experimentation to date, the chief characteristicrequired to be possessed by the gel strengthening agent is the small,spherical shape enabling it to react readily with the matrix binderand/or aggregate constituents.

As earlier noted, the moldable ceramic composition may advantageouslyfurther comprise refractory ceramic fibers, such as Kaowool™, Fiberfax™and Fiberkal™ type aluminosilicate fibers, Saffil™ alumina fibers,silicon carbide whiskers and calcium silicate fibers, to give furtherrigidity to the molded and fired filter structure. Typically, thesefibers can be present in an amount up to as much as about 60 percent byweight of the composition, but most typically are employed in amountsfrom about 1 to 4% by weight.

In the present invention, the components of the ceramic composition areselected to yield a particular setting time (e.g., by variation inaluminate/silicate ratio and/or solids content, and taking into accountthe temperature at which the composition will be cast or extruded),consistent with the anticipated duration of the foaming process in themold or during extrusion. As noted earlier, a distinct advantage of theinvention is that the setting time can be arranged to achieve aparticular dimensionally stable degree of gelation at or very near thetime when the gassing reaction ceases, thus insuring retention of thedeveloped porosity in the molded and eventually fired article. Ifgelation occurs too soon, the composition lacks the freedom to developand accommodate the desired degree of porosity and/or may result incracking of the set structure as gas continues to be evolved, while ifgelation is delayed too long, the developed porosity will have atendency to break down before the structure can be firmed up. While thislatter problem might be curable by excess utilization of surfactantand/or aluminum, cure in this way may introduce into the article toosubstantial amount of components making control more difficult and whichmay adversely affect final product characteristics.

As noted, the presence of silica fume in the composition results insubstantial reduction of the viscosity of the composition, the measuredreduction being greater at higher spindle speeds on the measuring deviceand also greater with increasing amount of silica fume. The greenstrength (as measured by the modulus of rupture or MOR) of the moldedshapes generally increases with increasing silica fume content. Increasein the amount of surfactant or increase in available surface area ofmetal (aluminum) (increase in amount or also, e.g., by using either aflaked metal or smaller grain size) increases the number of pores perlinear inch in the molded product. Increase in slurry temperature orother means to decrease set time results in an increase in density ofthe molded product, while a decrease in the available surface area ofaluminum or other metal powder also increases the density.

Following the removal of the molded porous ceramic shape from the moldor extrusion chamber, it is necessary to treat it to reduce or, ideallyeliminate, alkali metal (e.g., sodium) therein prior to the firingprocess so as to avoid the formation in the fired filter of glassyphases which would reduce the refractoriness or thermal shock stabilityof the ceramic filter. This may be accomplished by a number oftechniques, but the most preferred is to contact the unfired porousshape with water to leach alkali metal compounds therefrom, and then tofollow this with contact with a dilute aqueous solution of an ammoniumsalt, such as ammonium chloride, to effect substantially completeexchange of ammonium ion for any sodium ion remaining.

Following any removal of alkali or other fluxing or glass-formingingredients, the molded article is dried to remove any water therefromand is then fired in any suitable furnace at the temperatures required(e.g., 2200° F. to 2600° F.) to form the shaped porous ceramic filterarticle. Depending upon the composition of the moldable ceramiccomposition and the processing conditions, sintered ceramic refractoryarticles can be prepared having a broad range of porosity, surface areaand the like.

A wide range of refractory foam compositions can be achieved using thebasic procedures outlined above depending on the specific requirementsof the final ceramic filter product. For example, if thermal shockresistance is of paramount importance, refractory compositions thatresult in low thermal expansion can be incorporated such as thosecontaining lithium aluminosilicate, cordierite (a magnesiumaluminosilicate) and/or aluminum titanate. In addition, if strength andtoughness are more important, then such materials as mullite,zirconia-toughened ceramics and ceramic composites may be incorporated.If high thermal conductivity is important, then the use of siliconcarbide or silicon nitride is recommended. If high refractoriness isimportant, pure alumina can be used. If long term durability is requiredin both thermal and mechanical shock conditions, then low expansion,strong and tough type systems will be utilized.

As noted earlier, the preferred embodiment of this invention is a porousceramic filter element which, on its outlet surface, is provided with athin porous ceramic membrane layer of predominant open-celled porositywhich serves as a final filter and enhances the overall filteringcapacity of the filter without significant increase in clean backpressure. The ceramic membrane layer is arranged to have pores which aresmaller than those of the remaining portions of the filter element.

A wide variety of means can be used for providing this membrane on theporous, refractory aluminosilicate-based ceramic filter elementaccording to the invention.

In one such method, the porous molded ceramic shape, after formation butprior to firing, is treated by applying to one or more surfaces or areasthereof which will, e.g., constitute filter outlet surfaces in the finalfired ceramic filter, a ceramic paste or slurry containing a fugitiveconstituent capable of leaving a small pore when removed during thedrying or firing operation. The fugitive constituent can be a sublimablecompound or a burnable (e.g., carbonaceous) compound, utilized in a sizeand an amount which will result in pores having an average diametersmaller than that of the pores which will be present in the body portionor at untreated surfaces. During the firing operation, the ceramic pasteor slurry becomes integrally associated with (fused to) the porous bodyportion.

In another method, surfaces of the mold corresponding to the areas onthe part which will be, e.g., the outlet surface in the final firedceramic filter, are treated by application thereto (generally onto themold release agents already present) of a mixture of ceramic powder andfugitive constituent. The composition is then poured or injected intothe mold and, after setting and removal from the mold, will haveassociated with it at the areas corresponding to the pre-treated moldsurfaces, a thin skin of ceramic material which is rendered porousduring the firing step. In this embodiment, it is also possible toeliminate use of fugitive constituents by choosing for the ceramicpowder ingredients which are more refractory than those of theunderlying body portion, such that during firing, the greaterrefractoriness of these grains prohibits sintering thereby leaving apartially-sintered, i.e., porous, membrane layer on the preselectedareas of the body portion.

Among the preferred methods according to the invention involves theapplication of a ceramic paper (either woven, air-laid, or the like)atop the release agent on the appropriate mold surfaces prior to castingor injection-molding of the ceramic composition. In this manner, thecomposition, during foaming, expands into the ceramic paper, therebylaminating or bonding the systems together. On firing, there isdeveloped a porous body portion having on one or more of its surfaces athin porous ceramic membrane layer by reason of the nowintegrally-bonded ceramic paper whose pores are on the average smallerthan those of the underlying body portion.

In the most preferred methods, formation of a porous ceramic membranelayer is accomplished integral with the formation of the underlyingporous body. In situ processing in this manner offers significantadvantage in the economics of manufacture of the final ceramic filterarticle.

According to one of these preferred methods, the release agent used inthe mold, at the appropriate areas corresponding to where on the finalceramic filter the outlet surfaces will be, consists of or contains adefoaming surfactant (i.e., a foam suppressor). During the internaldevelopment of porosity in the molded composition by virtue ofgas-generating reactions therein, the defoaming agent acts tosufficiently suppress the reaction to keep the pores at these surfacescontrollably small, i.e., smaller than those within the body portion andat surfaces not in contact with the foam suppressor. Since thesurfactant is per se a release agent or is associated with a releaseagent, no problems are encountered in demolding the part. Commonly usedsurfactants for the defoaming of detergents, paints, varnishes and thelike are eminently suitable for this purpose.

According to another such preferred method, there is used, as therelease agent per se or along with a release agent, a foam suppressingagent consisting of an organic compound having an unhindered hydroxylgroup (i.e., an OH-"tail"), such as common alcohols, polyethyleneglycol, polyvinyl alcohol, and the like. By provision of such agents onmold surfaces corresponding to those areas of the body portion where theporous ceramic membrane layer is desired, the hydroxyl group apparentlyabsorbs the outgassing hydrogen molecules at these surfaces, therebyrestricting their growth. A porous ceramic membrane is attained byvirtue of the underlying foaming reaction and the fact that hydrogen gasbubbles at the desired surfaces are kept small.

In another method application to this aluminosilicate system, moldsurfaces corresponding to those where a porous ceramic membrane isdesired to be formed on the final ceramic filter product are providedwith a gel accelerating agent, preferably along with a release agent,and most preferably along with a release agent consisting of orcontaining an OH-tail as above described. The gel accelerating agentserves to locally set the aluminosilicate hydrogel prior to reactionbetween the particulate metal and alkali compounds in the castingcomposition with the result of formation of a thin porous ceramicmembrane layer having open pores which are smaller than those of theremaining portions of the filter.

Additional methods to achieve localized rapid gelation of thealuminosilicate system at surfaces where a porous ceramic membrane isdesired include incorporation of water along with the release agent atthe desired mold surfaces, the water being in an amount such that thecombined, but not yet set, silicate and aluminate mixture absorbs asufficient portion of this water to locally dilute the original amountsof soluble silicate and soluble aluminate, thereby locally reducing thegel time at these surfaces as compared to that occurring throughout theremainder of the composition. In another method, it can be arranged thatwater is locally removed from surfaces where a porous ceramic membraneis desired so as to bring about more rapid gelation of thealuminosilicate system at those areas (by virtue of increased solidsconcentration). This can be achieved, for example, by treating thecorresponding mold surfaces with a hydroscopic release agent (or arelease agent composition containing a hydroscopic agent) or byarranging a layer of dry paper at the required mold surface or bylocalized heating of the required mold surface.

Another method applicable to the aluminosilicate hydrogel system is tobring about a change in pH on the surface where the porous ceramicmembrane is required. For example, incorporation of an acidic componentin the release agent such as acetic acid or dilute hydrochloric acidwill locally accelerate the gelation prior to the onset of foaming.

To further describe the present invention, a number of examples arepresented in the following section illustrating a variety of potentialcompositions, processing techniques and the like. In accumulating thedata set forth in the examples, density, three-point modulus of rupture(MOR) and the coefficient of thermal expansion were measured by standardASTM techniques. The pore structure (number of complete pore cells perlinear inch) was measured using both Scanning Electron Microscope (SEM)and stereographic light microscope photographs. The predominantmicrostructural phases were determined using X-ray diffractiontechniques as well as light microscope observation of polished andetched surfaces. The relative permeability was obtained using a turbinetype air blower and recording the back pressure associated with the opencelled structure as 100 scfm (standard cubic feet per minute) of air wasforced through. Thermal shock resistance was interpreted as the percentof initial MOR strength the ceramic foam retained after exposing thematerial to 100 thermal cycles between room temperature and 1250° F.

EXAMPLE 1

A particularly effective ceramic foam filter for high temperaturefiltering applications was prepared using fused cordierite and calcindkyanite (mullite) refractories in the following manner. Initially, twoslurries were prepared, one containing sodium silicate and the other,sodium aluminate. The slurries were prepared to a specific gravity of2.1 g/cc at a viscosity of 25,000 cps at 70° F.

    ______________________________________                                        Sodium Silicate Slurry                                                        ______________________________________                                        sodium silicate grade 50 (44.1% solids)                                                               27.2%                                                 additional process water                                                                              5.4%                                                  Dow surfactant DC 190 (tradename)                                                                     0.6%                                                  silica fume (1/4 micron)                                                                              1.6%                                                  chopped fibers (1/8 and down)                                                                         2.0%                                                  fused cordierite (-200 mesh)                                                                          30.2%                                                 calcine kyanite (-200 mesh)                                                                           32.7%                                                 powdered aluminum metal (6-9 micron)                                                                  0.3%                                                  ______________________________________                                    

    ______________________________________                                        Sodium Aluminate Slurry                                                       ______________________________________                                        sodium meta-aluminate solution (46% solids)                                                             25.9%                                               additional water          5.7%                                                Dow surfactant DC 190 (tradename)                                                                       0.6%                                                silica fume (1/4 micron)  1.5%                                                chopped fibers (1/8 and down)                                                                           1.9%                                                fused cordierite (-200 mesh)                                                                            33.9%                                               calcined kyanite (-200 mesh)                                                                            31.0%                                               ______________________________________                                    

Using a high shear type mixer or blender apparatus, equal weights (360g) of the above slurries were combined and cast into a mold cavity withan 840 cc capacity. Since the slurries had a specific gravity of 2.1g/cc, only 41% of the mold cavity was filled. In approximately 30seconds after the two slurries were combined at 70° F., the mix began tofoam with a predominantly open-celled structure thereby filling the moldto yield a 0.86 g/cc wet density. Foaming stopped when the sodiumaluminosilicate hydrogel binder phase "set" (approximately 3-4 minutes),freezing the expanded structure in place. The hydrogel bond developedsufficient strength in 8-10 minutes to allow the cast part to bedemolded.

At this point the part contained 4.6% sodium oxide and 20.1% water atthe above mentioned 0.86 g/cc density. In order to increase therefractoriness, the sodium oxide was then removed. This was accomplishedby rinsing the part, in this case a 10 inch diameter plate, 5/8 inchesthick, with 10 liters of purified water (preferably deionized water witha 50,000 ohm resistance or better). This rinse reduced the sodium oxidecontent to approximately 2%, the stoichiometric amount. To remove theremaining sodium, the part was then subjected to 30-40 liters of a 1%ammonium chloride solution whereby substantially all of the NH₄ ⁺ ionsreplaced the Na⁺ ions. An additional 5 liter water rinse was thenperformed to remove excess Cl⁻ ions after which the part was removed andallowed to drain and dry.

After the initial draining and air drying period, the part was heated ina vented oven to 400°-600° F. to further dehydrate and remove some ofthe ammonium present. The length of time the part is in the oven varieswith the particular type of oven (conventional or microwave) and thespecific amount of water and ammonium in the part as well as the part'sporosity. In this particular example, the 10 inch plate was heated to600° F. in 6 hours. The warm part was removed from the oven and placeddirectly in a kiln supported on suitable kiln furniture. The part wasthen slowly heated to the required firing temperature of 2425° F. in10-12 hours. Once at temperature, the part was held for 2 hours tocomplete the sintering operation before being allowed to furnace cool.

Using the above formulation and process, the 10 inch ceramic foamedplate was characterized as follows:

    ______________________________________                                        Density            0.63 g/cc                                                  Sodium content     less than 0.5%                                             Permeability (in a ten-                                                                          4 inches of water                                          inch plate)        back-pressure at 100                                                          scfm                                                       Predominant microstructure                                                                       cordierite, mullite                                        Coefficient of expansion                                                                         1.5 × 10 (-6) to 700° C.                                         3.2 × 10 (-6) to 1000° C.                     Modulus of rupture 400-450 psi                                                Modulus of rupture after 100                                                                     380-410 psi                                                cycles from RT to 1250° F.                                             Pore structure     30 pores per linear                                                           inch                                                       Apparent refractoriness                                                                          2500° F.                                            ______________________________________                                    

A series of 10 inch reticulated foamed plates produced in this mannerwere placed in the exhaust stream from a 1982, 6.2 liter diesel engineto determine their effectiveness in filtering diesel particulatesproduced. The plates were arranged in a "stacked element" design. Whilethe exact collection efficiency was not determined, a considerableamount of particulates were trapped within the cross-sectional area ofthe plates. Once filled with particulates, the plates were regeneratedby placing them in a furnace and heating to the soot ignitiontemperature of 960° F. at which time the plate increased in temperatureover that of the surrounding furnace environment indicating anexothermic reaction or burning of the particulates was taking place.

After regeneration, the plates were subjected to the same air flowpermeability test to determine how much of the particulates or soot hadbeen removed. Since the same 4 inches of water backpressure was reached,it was assumed that all particulates were burned.

The plates were put back in the exhaust stream to collect moreparticulates, but this time regeneration was performed using a dieselfuel burner/blower arrangement that was able to heat the ceramic platesin 3-4 minutes to 1400° F. thereby setting up a more thermal shock proneregeneration cycle that was more in tune with the expected serviceenvironment. Again, the regenerated plates experienced the samebackpressure resistance as new plates. After a number of suchregeneration cycles were performed, the plates were sectioned into MORtest bars and broken to determine if any loss in strength occurred asthe result of such thermal cycling. No significant loss in strength wasrecorded.

EXAMPLE 2

An essentially all cordierite type ceramic foam was produced in asimilar manner to that in Example 1 is with fused cordierite aggregatebeing the primary aggregate in each of the two reactive slurries, i.e.,silicate and aluminate slurries, as follows:

    ______________________________________                                        Silicate Slurry                                                               ______________________________________                                        sodium silicate grade 50                                                                              27.1                                                  additioal water         6.0                                                   Dow DC 190 (tradename) surfactant                                                                     0.6                                                   silica fume             3.6                                                   fused cordierite (-200 mesh)                                                                          60.8                                                  powdered aluminum metal (6-9 microns)                                                                 0.1                                                   chopped fibers          1.8                                                   ______________________________________                                    

    ______________________________________                                        Aluminate Slurry                                                              ______________________________________                                        sodium meta aluminate solution                                                                       24.7                                                   additional water       6.9                                                    Dow DC 190 (tradename) surfactant                                                                    0.6                                                    silica fume            3.2                                                    fused cordierite (-200 mesh)                                                                         62.8                                                   chopped fibers         1.8                                                    ______________________________________                                    

The above slurries were blended together as in Example 1 with the samevolume expansion due to the foaming action of the aluminum metal andsodium hydroxide portion of the binder. Rinsing, ion-exchange and dryingwere also performed as they were in Example 1. Firing however, wasslightly lower with an upper holding temperature of 2000° F.

The above formulation and procedures resulted in a ceramic foam with thefollowing properties and characteristics:

    ______________________________________                                        Density              0.8 g/cc                                                 Pores per linear inch                                                                              ˜30                                                Coefficient of thermal expansion                                                                   1.8 × 10.sup.-6 in/in °C.                   MOR                  827 psi                                                  MOR after 100 thermal cycles                                                  (to 1250° F.) 777 psi                                                  Permeability (backpressure at                                                 100 scfm)            40 inches H.sub.2 O                                      Principal microstructural                                                     constituent          Cordierite                                               ______________________________________                                    

EXAMPLE 3

The same mix and procedures of Example 2 above were prepared with theexception that the final firing temperature was reduced to 1800° F. Thepurpose in firing to a lower temperature was to increase thepermeability (lowering the backpressure at the 100 scfm flow rate) ofthe final foamed ceramic. Using the lower firing temperature resulted inthe following properties and characteristics:

    ______________________________________                                        Density              .74 g/cc                                                 Pores per linear inch                                                                              ˜30                                                Coefficient of thermal expansion                                                                   1.8 × 10.sup.-6 in/in °C.                   MOR                  505 psi                                                  MOR after 100 thermal cycles                                                  (at 1250° F.) 469 psi                                                  Permeability (backpressure at                                                 100 scfm)            18 inches of water                                       Principal microstructural                                                     constituent          Cordierite                                               ______________________________________                                    

EXAMPLE 4

An essentially all calcined kyanite mix was also produced in a similarmanner of mixing, leaching/rinsing, ion exchange and drying as that setforth in the above examples using the following slurry formulations:

    ______________________________________                                        Silicate Slurry                                                               ______________________________________                                        sodium silicate grade 50 26.0                                                 additional water         5.8                                                  Dow DC 190 (tradename) surfactant                                                                      0.6                                                  silica fume              3.3                                                  calcined kyanite (-200 mesh)                                                                           62.5                                                 powdered aluminum metal (6-9 microns)                                                                  0.1                                                  chopped fibers           1.7                                                  ______________________________________                                    

    ______________________________________                                        Aluminate Slurry                                                              ______________________________________                                        sodium meta aluminate solution                                                                       23.7                                                   additional water       6.6                                                    Dow DC 190 (tradename) surfactant                                                                    0.6                                                    silica fume            3.0                                                    calcined kyanite (-200 mesh)                                                                         64.5                                                   chopped fibers         1.6                                                    ______________________________________                                    

Once properly dried, the calcined kyanite foamed plates were fired at2600° F. with the following properties and characteristics obtained:

    ______________________________________                                        Density              0.7 g/cc                                                 Pores per linear inch                                                                              ˜30                                                Coefficient of thermal expansion                                                                   4 × 10.sup.-6 in/in °C.                     MOR                  451                                                      MOR after 100 thermal cycles                                                  (to 1250° F.) Failed after 6 cycles                                    Permeability (backpressure at                                                 100 scfm)            12 inches of water                                       Principal microstructral                                                      constituent          Mullite                                                  ______________________________________                                    

EXAMPLE 5

A ceramic foam composition containing dispersed zirconium oxide as atoughening aid was also prepared using the sodium aluminosilicatehydrogel system and a fused zirconia-mullite aggregate using thefollowing starting slurry formulations:

    ______________________________________                                        Silicate Slurry                                                               ______________________________________                                        sodium silicate grade 50                                                                              21.9                                                  additional water        4.9                                                   Dow DC 190 (tradename) surfactant                                                                     0.5                                                   silica fume             2.8                                                   fused zirconia-mullite (-200 mesh)                                                                    68.4                                                  powdered aluminum metal (6-9 microns)                                                                 0.1                                                   chopped fibers          1.4                                                   ______________________________________                                    

    ______________________________________                                        Aluminate Slurry                                                              ______________________________________                                        sodium meta aluminate solution                                                                         19.9                                                 additional water         5.5                                                  Dow DC 190 (tradename) surfactant                                                                      0.5                                                  silica fume              2.5                                                  fused zirconia-mullite kyanite (-200 mesh)                                                             70.2                                                 chopped fibers           1.4                                                  ______________________________________                                    

Once properly dried, the foamed plates were fired at 2600° F. with thefollowing properties and characteristics obtained:

    ______________________________________                                        Density              .84 g/cc                                                 Pores per linear inch                                                                              ˜30                                                Coefficient of thermal expansion                                                                   5 × 10.sup.-6 in/in °C.                     MOR                  476 psi                                                  MOR after 100 thermal cycles                                                  (to 1250° F.) 395 psi                                                  Permeability (backpressure at                                                 100 scfm)            25 inches of water                                       Principal microstructural                                                     constituent          mullite/zirconia                                         ______________________________________                                    

EXAMPLE 6

Another useful ceramic foamed system based on a cordierite-siliconcarbide blend was also prepared using the sodium aluminosilicatehydrogel system and the same leaching/rinsing, ion exchange and dryingprocedures set forth in the above examples. The silicate and aluminateslurries used for this example consisted of the following materials:

    ______________________________________                                        Silicate Slurry                                                               ______________________________________                                        sodium silicate to grade 50                                                                           27.4                                                  additional water        6.2                                                   Dow DC 190 (trademark) surfactant                                                                     .5                                                    silica fume             2.8                                                   fused cordierite (-200 mesh)                                                                          34.0                                                  silicon carbide (-200 mesh)                                                                           27.4                                                  powdered aluminum metal (6-9 microns)                                                                 0.1                                                   chopped fibers          1.6                                                   ______________________________________                                    

    ______________________________________                                        Aluminate Slurry                                                              ______________________________________                                        sodium meta aluminate solution                                                                       23.6                                                   additional water       6.8                                                    Dow DC 190 (trademark) surfactant                                                                    0.5                                                    silica fume            2.4                                                    fused cordierite (-200 mesh)                                                                         65.1                                                   chopped fibers         1.5                                                    ______________________________________                                    

Once properly dried, the foamed plates were fired at 2200° F. with thefollowing properties and characteristics obtained:

    ______________________________________                                        Density              .75 g/cc                                                 Pores per linear inch                                                                              ˜30                                                Coefficient of thermal expansion                                                                   2.5 × 10.sup.-6 in/in °C.                   MOR                  380 psi                                                  MOR after 100 thermal cycles                                                  (to 1250° F.) 335 psi                                                  Permeability (backpressure at                                                 100 scfm)            22 inches of water                                       Principal microstructural                                                     constituent          Cordierite, SiC                                          ______________________________________                                    

EXAMPLE 7

A particularly effective ceramic foam filter for the high temperaturefiltering of diesel particulates includes the casting of a sodiumaluminosilicate hydrogel bonded system containing powdered aluminummetal to create a reticulated porous plate in which one side of the moldsurface was coated with a release agent based on polyethylene glycol3350, polyvinyl alcohol, glycerine and water of the followingcomposition:

    ______________________________________                                        polyethylene glycol 3350                                                                          17.5%                                                     polyvinyl alcohol solution                                                                        12.5%                                                     glycerine           36.5%                                                     water               33.5%                                                     ______________________________________                                    

Once the silicate and aluminate containing slurries were combined andcast into the mold, the mix adjacent to the above release agent rapidlygelled, thereby preventing the growth of any hydrogen gas bubble thatmay have formed near the mold surface as the result of the reaction ofthe aluminum metal powder and the sodium hydroxide in the mix. Afterdemolding, the cast part displayed an excellent "skin" or smoothmembrane surface that upon further processing to remove sodium and waterfollowed by firing at a suitable temperature to form ceramic bonds,remained porous even though by naked eye the surface appears dense.

A series of 10 inch foamed plates produced in this manner with suchexcellent ceramic membrane surfaces on one side were fashioned into astacked element filtering arrangement. The efficiency of the dieselparticulate collection was measured using a suitable device at a majordiesel engine manufacturer's test facility and found to be in the 65-70%efficient range. Such efficiencies would make the 1991 and 1994 dieselengine prototypes now being designed meet the EPA emission standards.

EXAMPLE 8

Another particularly effective mold release that works in the sodiumaluminosilicate hydrogel system to create a porous membrane surfaceagain uses water as the gel accelerating ingredient and simply glycerinefor its release effects as follows:

    ______________________________________                                                glycerine                                                                            50%                                                                    water  50%                                                            ______________________________________                                    

Again, after the silicate and aluminate containing slurries werecombined and cast into the mold with the above release agent, a rapidgelation occurred against this surface creating a smooth skin free oflarge gas bubbles since the surfaces in question had already set priorto foaming.

EXAMPLE 9

In a similar manner, the same mold is coated on one surface with a thinlayer of woven mullite fibered paper. The sodium aluminosilicatehydrogel system is cast and allowed to foam in the normal manner. Afterdemolding, the woven paper will be significantly attached to the ceramicfoam so that no separation occurs during the subsequent processing toremove sodium and water prior to firing. The resultant laminatedstructure will exhibit excellent thermal shock and filtering efficiency.

EXAMPLE 10

A silicone release agent modified with a silicone defoaming surfactantwas sprayed on one side of a metal mold that was heated to 140°-150° F.The other side of the mold remained at room temperature. A sodiumaluminosilicate hydrogel bonded foaming mix such as that disclosed inExample 1 was then cast into the mold. Due to the accelerated set timeof the gel adjacent to the heated surface, insufficient time wasavailable for the foam cells to grow to any appreciable size. Thefurther away from the hot surface, the larger the cell walls were ableto grow prior to gelation. Even though the surface adjacent to theheated surface appeared smooth and dense to the naked eye, once the foamwas rinsed in deionized water and subjected to ionic exchange to removethe sodium from the hydrogel structure as disclosed herein, this samesurface is actually quite porous and provides an excellent ceramicmembrane effect.

Having described the invention with reference to particularcompositions, processes, examples and embodiments, it is to beunderstood that these particulars are presented for purposes ofillustration and description, and are not otherwise intended as strictlimitations upon the scope of the fundamental invention as defined inthe appended claims.

What is claimed is:
 1. A process for producing a porous refractoryceramic filter for use in the filtering of particulates from dieselexhaust gases, said filter having an inlet surface for receipt of dieselexhaust gases and an outlet surface for discharge of diesel exhaustgases which have traversed through said filter, comprising the stepsof:(a) providing a foamable ceramic composition comprised of an aqueousadmixture of (1) an alkali metal silicate and an alkali metal aluminate,each in an amount effective to form therebetween an aluminosilicatehydrogel which binds all components of said composition, after foaming,into a self-supporting body in the desired shape of said filter; (2)refractory ceramic materials; (3) metal powder in an amount effective toreact with alkali metal in said composition and generate as a producthydrogen gas in an amount effective to bring about foaming and expansionof said foamable ceramic composition; (4) a surfactant in an amounteffective to form from said hydrogen gas small bubbles which provide apredominantly open-celled porosity in the foamed and expanded foamableceramic composition, and (5) a gel strengthening and viscosity-modifyingmagnet in an amount effective to reduce the viscosity of said foamableceramic composition as compared to the same composition without saidgel-strengthening and viscosity-modifying agent; (b) introducing aquantity of said foamable ceramic composition into a shape-defining areafor a time sufficient to permit reaction among the components of saidcomposition to produce a foamed, porous aluminosilicate hydrogel-bound,self-supporting ceramic precursor of predominantly open-celled porosityin the shape of said shape-defining area; (c) removing saidself-supporting precursor from said shape-defining area; (d) contactingsaid self-supporting precursor with water for a time effective to removetherefrom leachable alkali metal compounds; (e) thereafter contactingsaid self-supporting precursor with a dilute solution of non-alkalimetal inorganic salt to effect substantially complete exchange of theion of said non-alkali metal inorganic salt for the alkali metal ion ofremaining alkali metal compounds in said self-supporting precursor; and(f) thereafter firing said self-supporting precursor to form ceramicbonds therein and produce said porous refractory ceramic filter having apredominantly open-celled porosity.
 2. The process according to claim 1further comprising providing on said outlet surface of said porousrefractory ceramic filter a thin, integral, porous ceramic membranelayer of open-celled porosity, the pores of which have an averagediameter less than that of the pores within and at said inlet and othersurface of said porous refractory ceramic filter.
 3. The processaccording to claim 1 wherein said alkali metal silicate and said alkalimetal aluminate are a sodium silicate and a sodium aluminate.
 4. Theprocess according to claim 3 wherein said gel strengthening andviscosity-modifying agent is silica fume.
 5. The process according toclaim 4 wherein said metal powder is powdered aluminum.